The present invention relates to a power transmission apparatus of a four wheel drive vehicle which includes a front wheel to which a driving force of an engine is directly transmitted, and a rear wheel to which a portion of the driving force of the engine is indirectly transmitted via a multiple disc clutch, and in which the multiple disc clutch is engaged by a hydraulic pressure generated by a hydraulic pump in accordance with a difference between rotational speeds of the front wheel and the rear wheel.
There is publicly known a power transmitting apparatus of such a four wheel drive vehicle provided with a torque cam mechanism for engaging a multiple disc clutch by a thrust force in an axial direction generated in accordance with a difference between rotational speeds of a first cam member moved in cooperation with rotation of a front wheel and a second cam member moved in cooperation with rotation of a rear wheel to promote response until a four wheel drive state is brought about after generating the differences between the rotational speeds of the front and the rear wheels by Patent Reference 1, shown below.
[Patent Reference 1]
JP-A-2001-213185
Meanwhile, although in running a vehicle forward at a constant speed, a rotational speed of a front wheel and a rotational speed of a rear wheel are to be the same, there is a case in which the rotational speed of the front wheel exceeds the rotational speed of the rear wheel by a difference in a small slip rate between the front and the rear wheels by a running resistance or the like, at this occasion, according to the power transmission apparatus of the four wheel drive vehicle described in Patent Reference 1, the multiple disc clutch is engaged to bring about an unnecessary four wheel drive state, further, a transmitting torque of the multiple disc clutch is amplified since the power transmission apparatus is provided with the torque cam mechanism and therefore, there poses a problem of increasing a fuel consumption amount of the engine. Further, a frequency of bringing about the four wheel drive state is increased and therefore, an extra strength needs to set in order to ensure durability of a drive system to pose a problem of causing an increase in weight of the drive system and an increase in cost.